I think cliffs should be harder to remove. Right now it's just a quick drop of some explosives. It would be interesting to instead have them creep as you blow them up. When you blow up a length of cliff, surrounding cliffs show up just ahead of the cliff you blew up. I can see this getting buggy as hell, but we have REALLY smart devs who can do anything.

For example a player should be able to slowly climb them with some special animation. Same goes for tanks, but not for cars or trains.
Belt and tube animations should change to reflect the cliff - like staircase belts or vertical tubes.
Also forests should get some shadows if they grow on a cliff.
Removing cliffs should be like in real life - very expensive, only with the most powerful explosive.

Keep up the good work guys, I know that soon factorio cliffs will be as awesome as everything else!